1. Technical Field of the Invention
The present invention relates to the field of magnetic resonance imaging (MRI) and, in particular, to a system and method for aiding the efficient design of pulse sequences.
2. Description of Related Art
Until the development of MRI and Nuclear Magnetic Resonance (NMR) technology by Dr. Raymond V. Damadian in the 1970's, diagnostic imaging of internal physiology was limited to techniques which provide limited soft tissue contrast. For example, as is well understood in the imaging art, computed tomography (CT) techniques depend on tissue density, e.g., soft tissue compared to bone, and usage of contrast media, e.g., barium, both affecting x-ray attenuation and detection. Although CT, at present, reveals better bone detail, MRI is far superior for most other soft tissues, illuminating the internal networks and pathways to physicians without the known deleterious effects of x-rays.
Although a full description of how MRI works is not necessary to the understanding of the subject matter of the present invention, a brief illustration of the physical principles involved is set forth below. In short, MRI is a diagnostic method for providing detailed specimen images through manipulation of atomic nuclei, specifically hydrogen, within a specimen tissue. A fundamental property of individual nuclear particles is that individual particles spin or rotate about their own respective axes. As is understood in physics, a spinning charged particle produces a magnetic moment directed along that particle's axis of rotation. These spinning nuclei and their resulting moments are randomly oriented in the absence of any external magnetic fields. However, by applying a magnetic field, the rotating nuclei essentially align their axes either in parallel or in opposition to the magnetic field. Those nuclei aligned in opposition to the magnetic field have a higher energy than those nuclei that are aligned in parallel with the field. A small majority of nuclei will be aligned in the lower energy state, i.e., in parallel, than opposed to the same field, usually only measuring in parts per million for the excess. By the addition of energy, e.g., by application of radio frequency (RF) energy, to these lower energy state excess nuclei, these nuclei can be transitioned to align themselves antiparallel or in opposition to the magnetic field. As is understood in the art, it is these few realigned nuclei that ultimately provide the information used to generate an MRI image.
While the respective nuclei are generally aligned with the applied magnetic field, it should be understood that this alignment is not precisely with a plane parallel to the axis of the magnetic field. Instead, the nuclear moments align at a slight angle from the axis of the magnetic field and precess about this axis. This frequency of precession, along with the magnetic moment caused by the alignment of the nuclei, comprise the phenomenon on which imaging by magnetic resonance is based.
The frequency of this atomic or nucleic precession, also referred to as the Larmor frequency, is a function of the specific nucleus and the strength of the external magnetic field. The nuclei will absorb energy and induce a signal in adjacent RF receptor coils only at the particle's Larmor frequency—an event referred to as “resonance.” In other words, by applying energy to the specimen at the Larmor frequency, the net magnetic moment of the excess nuclei may be reversed, or deflected, to the opposite or antiparallel direction by causing these parallel state particles to elevate to the higher energy state. The radiofrequency energy pulses applied are referred to as “excitation pulses.” The duration of the RF pulse specifies the duration of the nuclear moment deflection. When the excitation pulse is removed, the nuclei will then begin to lose energy, causing the net magnetic moment to return to its original, lower energy state orientation, and the energies emitted during this transmission are used to create the image of the specimen.
Present day MRI devices generally scan only hydrogen atoms. The hydrogen atom is most attractive for scanning since it comprises the largest atomic percentage within the human body and provides the largest magnetic resonance (MR) signal respective to other elements present in human organs. As described hereinabove, every nuclear particle spins about its axis and the individual properties of the spin are defined by the specific nuclear particle in question, e.g., hydrogen, creating a magnetic moment with a defined magnitude and direction. The Magnetic Resonance (MR) signal itself is a complex function dependent upon the concentration of the deflected hydrogen atoms, spin-lattice relaxation time (T1), spin-spin relaxation time (T2), motion within the sample and other factors as is understood in the art.
Another component of the MR signal is, of course, the particular series of RF and magnetic field gradient pulses employed in the form of pulse sequences. Varying the pulse sequences can produce considerable image differences, such as T1 emphasis (T1-weighted), T2 emphasis (T2-weighted), proton density emphasis or combinations thereof. Common sequences include Gradient Echo (GE), Spin Echo (SE), Inversion Recovery (IR), Double Spin Echo, 3-dimensional Gradient Echo (3DGE), 3-dimensional Spin Echo (3DSE), Fast Spin Echo (FSE), Partial Saturation (PS) and others. It is understood that these sequences are illustrative only and the present invention is in no way limited to application of only these specific sequences. Since one sequence image type may not optimally illustrate an area of consideration, multiple images using varying sequences of pulses may be required to fully analyze the area, as is understood in the art.
At present, conventional MRI systems offer fairly primitive interfaces for the design of the aforementioned pulse sequences. In particular, present MRI systems are ill-suited for sequence designers who must input and modify customized pulse sequences. Furthermore, this input is generally made by coding the sequence in a programming language, e.g., C, and is further complicated in that the coded sequence format must be tailored for each individual machine, thus necessitating that the sequence designer must be skilled in the programming arts along with the MRI technologies or alternatively requiring an MRI sequence designer to work in coordination with a programmer. Consequently, conventional systems generally lack real-time communications with the MRI scanner since each sequence must first be coded and compiled prior to being loaded on the system.
Accordingly, a first object of the present invention is to provide an improved MRI apparatus for more efficient creation and development of MRI pulse sequences.
It is a second object of the present invention to provide a graphical user-interface for performing the mathematical calculations related to MRI pulse sequence design, thereby providing an automatic graphical response of the interface to user manipulation, facilitating interaction between the user and the interface.
It is a third object of the present invention to provide a graphical user-interface for intermediating between the sequence designer and the MRI hardware such that the designer can directly view the details of the entire pulse sequence but can also access and modify the sequences directly through a mouse or keyboard.
It is a fourth object of the present invention to provide a real-time interface, or front-end, between the graphical user-interface and the MRI system hardware that enables real-time communication and interaction between the sequence creator and the MRI system hardware.
It is a fifth object of the present invention to provide real-time communication and interaction between the sequence creator and the MRI system hardware, enabling data acquisition and graphical display of the RF shapes, gradient waveforms and MRI signals received inside the magnetic field and providing analysis of this information in real-time.
It is a sixth object of the present invention to provide a real-time communication and interaction between the sequence creator and the MRI system hardware, thereby enabling dynamic manipulation of the details of a MRI pulse sequence accessed through the graphical user-interface.
It is a seventh object of the present invention to provide a real-time communication and interaction between the sequence creator and the MRI system hardware that enables detection of dynamic deficiencies of the MRI system through feedback information and possible compensation for the deficiencies through sequence manipulation.
It is an eighth object of the present invention to provide a foundation for development of automated calibration of imaging sequences of an MRI system.